Liquified natural gas (LNG) has been proposed as a fuel in transportation vehicles such as automobiles and buses. However, use of LNG in transportation vehicles presents a number of problems that have not previously been satisfactorily solved.
LNG is a cryogenic liquid stored under saturated conditions. Therefore, all materials in contact with it must be able to perform satisfactorily at cryogenic temperatures in the range of approximately 90 to 190 degrees Kelvin, depending upon the storage pressure. This includes the pump for transferring the LNG from the storage reservoir to the engine.
Although LNG pumps are well-known, such pumps are usually used in process plants where the pumps are under relatively continuous monitoring. By contrast, LNG pumps in transportation service must be able to perform reliably for long periods of time (up to several years) under start-stop conditions without continuous monitoring and without excessive maintenance. This is difficult since LNG pumps operate without lubrication other than the LNG, which has relatively low lubricity. Such pumps must also be able to operate at high discharge pressure but low flow rates, which increases the difficulty of maintaining low rates of leakage past the check valves and piston rings.
In addition, because LNG is a saturated liquid, pumps must have very low suction pressure requirements to prevent cavitation. These requirements are difficult to meet in any reciprocating pump and, in transportation vehicles, the available suction pressure may be particularly low due to the limitations of size and placement of the LNG storage reservoir.
Although some efforts have been made to meet some of the requirements of LNG pumps on transportation vehicles, such efforts have not satisfactorily solved all the problems in a cost-effective manner. For example, in one cryogenic pump, a ceramic coating on the inside surface of the cylinder sleeve has been proposed to minimize wear on the cylinder sleeve during use in an effort to meet the reliability requirements. However, ceramic coatings are relatively expensive. Furthermore, ceramic coatings can flake off, presenting problems.
Accordingly, there is a need for an LNG reciprocating pump that can meet high standards for reliability and performance, particularly under low suction pressure, high discharge pressure, low flow rate conditions, and can meet these standards in a cost effective manner.